1. Field of the Invention
The present invention relates to wireless-power transfer, and more specifically, the present invention relates to a wireless-power-transfer system that compensates for variations in a distance between a transmit coil and a receive coil of a resonant wireless-power-transfer system to provide a high-power transmission efficiency.
2. Description of the Related Art
Wireless-power-transfer systems have been used to provide power to devices without the need for direct and physical connections, such as cables or wires. Various methods have been used for transmitting power wirelessly, including capacitive coupling, resonant inductive coupling, laser light, and microwave beams. Wireless-power-transfer systems that use resonant inductive coupling provide relatively efficient power transfer from a transmit coil to a receive coil at relatively short coil-separation distances. More specifically, resonant inductive coupling is less expensive than methods such as laser light and microwave beams, and also provides advantages over simple magnetic induction coupling. Simple magnetic induction coupling has been used, for example, to charge electric toothbrushes or cellular phones placed directly upon a charging mat, but only allows for very limited coil-separation distances of less than one coil diameter.
In a loosely-coupled resonant wireless-power-transfer system, it is desirable to be able to move the transmit and/or the receive coils over a wide range of distances without requiring that the system resonances be constantly retuned. A resonant wireless-power-transfer system is considered to be loosely coupled if a majority of the flux generated by the primary coil is not received by the receiving coil. Taking into account the variation with distance of the coupling coefficient K between the transmit and the receive coils and the variation of the output voltage of the nonlinear reverse-biased capacitance of the rectifiers in the receiver, a specific tuning can be established that allows for coil-separation distances over a range of one to five coil diameters without retuning.
The coupling coefficient K indicates the proportion of flux from the transmit coil that penetrates the receive coil and is generally related to: (1) the mutual inductance of the transmit and receive coils when they are coupled to each other and (2) the self-inductance of each of the transmit and receive coils when the transmit and receive coils are standalone, uncoupled coils. The coupling coefficient K typically has a value between zero and one, with a value of zero indicating that there is no mutual flux, and thus no mutual inductance, between the transmit and receive coils. A value of one for the coupling coefficient K indicates that all of the flux from the transmit coil is received by the receive coil, and thus the self-inductance of each of the transmit and receive coils is the same as the overall mutual inductance between the transmit and receive coils. However, the coupling coefficient K may also have a negative value, for example, if the polarity of one of the transmit and receive coils is reversed, such that the voltage induced in the receive coil is 180 degrees out of phase with respect to the voltage in the transmit coil. Further, it is difficult to precisely determine the coupling coefficient K between transmit and receive coils, as equations for determining the mutual and self-inductances of the transmit and receive coils are complex, and measurements made to determine the coupling coefficient K are inaccurate, especially for low values of the coupling coefficient K.
One known method of retuning resonances in resonant wireless-power-transfer systems relies on varactors, which are a type of diode whose capacitance varies as a function of the voltage applied across its terminals. Varactors have been used as voltage-controlled capacitors in resonant wireless-power-transfer systems to deliver maximum power as the coil separation distance changes. However, varactors incur undesirable power losses in resonant wireless-power-transfer systems, and varactors often require complex control circuits or systems.
Another known method of retuning resonances in resonant wireless-power-transfer systems is to vary the operating frequency of the transmit and receive oscillators to maintain operation of the resonant wireless-power-transfer systems at their respective system resonant frequencies. However, if power-transfer frequencies are in the industrial, scientific, and medical (ISM) radio bands in which electromagnetic interference is internally permitted, very little bandwidth is available to allow for changes in the operating frequencies.
According to a known method, the voltage at the receiver output of a resonant wireless-power-transfer system can be limited by using larger value capacitors at the rectifier inputs to detune the receiver when an overvoltage condition occurs at the output, which helps to shunt the resonant tuning and power transfer between the transmit and receive coils to prevent excessive voltage gain. In particular, excessive voltage gain in the receiver may be prevented if a load of the receiver is small by forcing the wireless-power-transfer system to operate intermittently. However, this method causes power transfer to the receiver to stop, which reduces the amount of power that can be received.
According to another known method, the receiver output can be directly connected to a battery to prevent overloading of the receiver by forcing the receiver output voltage to be fixed to the battery voltage. Accordingly, a resonant wireless-power-transfer system including a battery at the receiver output is not subjected to constant re-tuning to attempt to receive the maximum amount of power available at the receive coil from the transmit coil every time the coupling coefficient K changes. More specifically, the battery presents a substantially fixed voltage for the load of the receiver, which provides a stable operating point on the power-transfer curve of the receiver output characteristics. However, this method forces the receiver output voltage to be fixed to the battery voltage, which is unlikely to be set at the maximum power-transfer point on the power-transfer curve of the receiver output characteristics.
Although the receiver could be tuned to provide a constant or substantially constant voltage output with respect to distance at the maximum power points, such an arrangement would prevent the receiver from providing power to the load if the power transfer is started at a relatively long coil-separation distance. More specifically, at a sufficiently high coil-separation distance, the receiver output voltage is close enough to zero volts that the rectifiers in the receiver are in an off state, and the reverse voltage across the rectifiers in this state is sufficiently close to zero, which increases the nonlinear receiver capacitance. Accordingly, the nonlinear receiver capacitance becomes too high to provide adequate gain to charge the output capacitor of the receiver to the operating voltage of the load.
According to yet another known method, an under-voltage lockout circuit can be added to the receiver output voltage such that if the receiver output voltage falls below a predetermined level, a DC-DC stage of the resonant wireless-power-transfer system is turned off. When the receiver output voltage rises above another predetermined level, the DC-DC stage is turned back on again. However, this method of operation results in intermittent operation of the wireless-power-transfer system, and only provides overload protection, and no tuning or re-tuning, for the resonant wireless-power-transfer system.
Thus, the above-described known methods result in the loss of efficiency in resonant wireless-power-transfer systems and/or require complex control circuits or systems.